Charge Volume Density To Charge Per Unit Length

[Baumer8993]

Homework Statement

http://imgur.com/VzZjSEH

I took a screen shot so I did not leave anything out.

Homework Equations

Surface area, and volume of a sphere.

The Attempt at a Solution

I am trying to find the linear charge density from the charge volume density. I got it explained from a friend. It was right, but made no sense. He did it by taking the volume density, and multiplying it by the charge volume density by pi, and (outer r² - inner r²).

So he did -657 * pi * (.042² - .026²)

Can someone explain to me how this is right?

 

[rude man]

It's the total charge per unit length due to the outer insulator. Charge = voume charge density x volume. The volume of the insulator is pi(r2^2 - r1^2) x 1 unit length. Multiply by the charge density rho and you have your answer.

Of course, the complete answer must include the linear charge density of the wire as well.

 

[vela]

Calculate the total charge Q of a length $l$ of the insulator. Then divide Q by $l$.